Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sdd.hp.com!zaphod.mps.ohio-state.edu!usc!apple!portal!atari!kbad
From: kbad@atari.UUCP (Ken Badertscher)
Newsgroups: comp.sys.atari.st.tech
Subject: Re: Malloc() and desk accessories
Message-ID: <2261@atari.UUCP>
Date: 28 Aug 90 19:28:55 GMT
References: <A2017203479@thelake.mn.org>
Organization: Atari Corp., Sunnyvale, CA
Lines: 42

As long as you don't go back into an event wait, you can Malloc all day
long (Mfreeing what you Malloc when you're done with it, of course).

The only time DA's have Malloc headaches if they try to Malloc memory
outside their startup sequence is if they give the parent application a
chance to terminate out from under them.  The problem arises when an
application calls appl_exit().  The AES does not give DA's enough time
to clean up after themselves before the parent application's resources
are released.  It is possible for a DA to not get an AC_CLOSE message
until you're already back at the desktop.

Note that this also causes problems with DA's that open virtual
workstations, since the VDI Mallocs memory for workstation information. 

A sequence of events like the following could cause problems:

  Run an application.
  Open a DA that opens a workstation or Mallocs some memory.
  Quit the application without closing the DA.

At this point, the application calls appl_exit(), which returns before
giving all DA's a chance to clean up.  The application then terminates,
and GEMDOS frees all the memory that was allocated while the program 
was running.  Eventually, the AES will get around to telling the DA,
"Oh, by the way, shut yourself down now."  If the DA then tries to
access any of the memory it had Malloced, it would be messing with
memory that had already been freed.  If this memory had since been
reallocated, the DA would be changing someone else's memory.  If it
happened that the memory had been reallocated to someone else _at the
same starting address_, it would be disastrous for the DA to Mfree that
block.

The solution is simple.  DA's must only Malloc memory in an atomic
(from the AES's point of view) operation, as Steve outlined.  If a DA has
more dynamic memory needs, it can allocate a heap for itself at
startup, BEFORE IT MAKES ANY AES CALLS.  The DA can then use it's own
memory management within this heap.

-- 
   |||   Ken Badertscher  (ames!atari!kbad)
   |||   Atari R&D System Software Engine
  / | \  #include <disclaimer>